I'd like to add that there are many false warnings for Fedhas: Freezing Cloud and Glaciate also produce warnings for example.

On the other hand Tornado and Shatter will damage your plant allies (probably other spells too), and make them hostile, but will not cost you any piety if they don't die. Once hostile Fedhas will still protect them from harm against the same spells/abilities as before, including ranged weapons. Only if they die do you lose a lot of piety, but are not placed under penance as long as your piety was high enough.